Hi Nick,  Thank you for your comment.  The fish sounds a bit large for a Common Jollytail and I think you are probably too far inland for this species.  Is it possible to take photos of a few of the fish and attach them to a new comment?  I am keen to try to put a name on your fish and if I can't, to forward images to an expert.

with this rain our dam has filled and over flowed. caught in the grass as the water has droped we found what we think are common jollytail, They range from 3cm to 15 cm and appear to have been swimming up stream, they must have been in the lower dam for a long time as the creeks here have not had any flow for at least 10 years. We are near Ballarat so a long climb from any coastal area if that is were they spawn. nicko0255